1. Field of the Invention
This invention relates to a process for producing Bi12MO20 particles. This invention also relates to a photo-conductor layer for constituting a radiation imaging panel, which photo-conductor layer utilizes the Bi12MO20 particles.
2. Description of the Related Art
A material represented by a chemical formula Bi12MO20, in which M represents Si or Ge, has photo-conductivity and dielectric characteristics. Therefore, it has heretofore been studied to utilize the Bi12MO20 material for constituting electro-photographic materials, X-ray detecting materials, ceramic capacitors, and the like. In cases where the Bi12MO20 material is to be utilized for constituting the electro-photographic materials, the x-ray detecting materials, the ceramic capacitors, and the like, Bi12MO20 particles have heretofore been produced by use of a solid phase technique, in which single oxides of the constituent elements are mixed together and fired. The solid phase technique for producing the Bi12MO20 particles is described in, for example, a paper by M. Valant and D. Suvorov, “Processing and Dielectric Properties of Sillenite Compounds Bi12MO20-δ(M=Si, Ge, Ti, Pb, Mn, B1/2P1/2)”, J. Am. Ceram. Soc., 84 (12), pp. 2900-2904, 2001.
However, the particles obtained with the solid phase technique often have the drawbacks in that the composition is not uniform, and in that the particle shapes and the particle sizes are not uniform. Therefore, with the particles obtained with the solid phase technique, it is not always possible to form a uniform molded material having a high density or a ceramic material having good quality. Also, in order for the solid phase technique to be performed, it is necessary to perform grinding and mixing steps. In the grinding and mixing steps, impurities originating from vessels utilized for the grinding and mixing steps inevitably mix into the particles. Therefore, the problems are encountered in that a finished product having sufficiently good performance is not capable of being obtained.
Besides the solid phase technique described above, a process for producing the Bi12MO20 material with a liquid phase technique has heretofore been known. As for the liquid phase technique, a technique for synthesizing Bi12MO20 is described in, for example, a paper by H. S. Horowitz et al., “Solution Synthesis and Characterization of Sillenite Phases, Bi24M2O40(M=Si, Ge, V, As, P)”, Solid State Ionics, 32/33, pp. 678-690, 1989. The technique for synthesizing Bi12MO20 described in the aforesaid paper comprises the steps of dissolving Bi(NO3)3 and an element source, which is selected from the group consisting of Na2O.xSiO2 acting as an Si source and GeO2 acting as a Ge source, in an acid, causing precipitation to occur by the addition of an alkali metal hydroxide, adjusting a pH value, and setting the temperature at an appropriate temperature, whereby Bi12MO20 is synthesized.
However, with the technique for synthesizing Bi12MO20 described in, for example, the paper by H. S. Horowitz et al., “Solution Synthesis and Characterization of Sillenite Phases, Bi24M2O40(M=Si, Ge, V, As, P)”, Solid State Ionics, 32/33, pp. 678-690, 1989, the produced Bi12MO20 particles have particle diameters of as large as approximately 10 μm. With the Bi12MO20 particles having the large particle diameters, the problems are encountered in that a dense molded material or a ceramic material having a high density is not capable of being formed. For example, the problems are encountered in that a photo-conductor layer produced by use of the Bi12MO20 particles having the large particle diameters has a low packing density and therefore has only a small effect of collecting generated electric charges.